Question

Assuming that the octet rule applies, draw the Lewis structure for each of these molecules. a. \(\mathrm{CCl}_{4}\) (carbon tetrachloride, a substance formerly used as a cleaning agent) b. \(\mathrm{H}_{2} \mathrm{O}_{2}\) (hydrogen peroxide, a mild disinfectant; the atoms are bonded in this order: \(\mathrm{H}-\mathrm{O}-\mathrm{O}-\mathrm{H})\) c. \(\mathrm{H}_{2} \mathrm{~S}\) (hydrogen sulfide, a gas with the unpleasant odor of rotten eggs) d. \(\mathrm{N}_{2}\) (nitrogen gas, the major component of the atmosphere) e. HCN (hydrogen cyanide, a molecule found in space and a poisonous gas) f. \(\mathrm{N}_{2} \mathrm{O}\) (nitrous oxide, "laughing gas"; the atoms are bonded \(\mathrm{N}-\mathrm{N}-\mathrm{O}\) ) g. \(\mathrm{CS}_{2}\) (carbon disulfide, used to kill rodents; the atoms are bonded \(S-C-S\) )

Short answer

Draw Lewis structures by assigning valence electrons, creating bonds, completing octets, and adjusting for multiple bonds if necessary.

Step-by-step solution

Understand the octet rule

The octet rule states that atoms tend to form molecules by sharing electrons so that each atom has eight electrons in its valence shell, resembling the electron configuration of a noble gas. Hydrogen is an exception, aiming for two electrons.

Assign valence electrons for each atom

Determine the total number of valence electrons available for each molecule. For instance, in \( \mathrm{CCl}_4 \), carbon has 4, each chlorine has 7, and there are 4 chlorine atoms, giving a total of \( 4 + (7 \times 4) = 32 \) valence electrons.

Draw the skeletal structure

Identify the central atom, usually the least electronegative (except hydrogen). For \( \mathrm{CCl}_4 \), carbon is the central atom, while chlorine atoms are terminal. Similarly, draw skeletal structures for each molecule, e.g., in \( \mathrm{H}_2 \mathrm{O}_2 \), an \( \mathrm{O}-\mathrm{O} \) bond, each \( \mathrm{O} \) linked to an \( \mathrm{H} \).

Distribute electrons to satisfy the octet

Allocate electron pairs (bonds) between atoms to satisfy the octet rule for as many atoms as feasible. Start by placing pairs of electrons between bonded atoms. For \( \mathrm{CCl}_4 \), place one single bond (two electrons) between carbon and each chlorine.

Complete octets with lone pairs

After satisfying bonded pairs, distribute remaining valence electrons as lone pairs to satisfy the octet for terminal atoms. In \( \mathrm{CCl}_4 \), add 6 lone pairs around each chlorine to complete the octet.

Account for multiple bonds if necessary

If the remaining electrons are insufficient to meet all atom octets, consider forming double or triple bonds. For \( \mathrm{N}_2 \), with 10 valence electrons, a triple bond between the two nitrogen atoms completes the octets.

Final Step: Verify and adjust if needed

Ensure all atoms satisfy the octet (or duet) rule. Re-check electron counts and distribution. For \( \mathrm{HCN} \), carbon and nitrogen share a triple bond, hydrogen forms a single bond with carbon, aligning with the octet rule.

Key concepts

Octet Rule

The octet rule is a fundamental principle in chemistry that helps us understand how atoms bond to form molecules. It suggests that atoms are most stable when they have eight electrons in their valence shell. This configuration mimics the electron arrangement of noble gases, which are known for their stability. However, there is a notable exception to this rule: hydrogen, which requires just two electrons to achieve stability like helium. To illustrate this, consider carbon tetrachloride (\(\mathrm{CCl}_4\)). Here, carbon forms bonds with four chlorine atoms, using its four valence electrons to share with chlorine, which itself has seven valence electrons. By forming a single bond with each chlorine atom, carbon achieves a full octet, while each chlorine completes its own octet as well. The octet rule is a guiding principle, and understanding it will help students predict molecular structures and reactivity.

Valence Electrons

Valence electrons are the outermost electrons of an atom. They are crucial because they participate in chemical bonding and determine an atom's chemical properties. In molecular structures, accounting for valence electrons allows us to understand how atoms connect and how they fulfill the octet rule.
For example, in the molecules given in the exercise, knowing the number of valence electrons is the first step in drawing Lewis structures.

• Carbon has 4 valence electrons.
• Each chlorine atom has 7 valence electrons.
• Oxygen has 6, and hydrogen has 1 valence electron.
• Nitrogen has 5 valence electrons.

Summing up the valence electrons properly is paramount to ensure we accurately distribute these electrons to satisfy the octet rule.

Molecular Geometry

Molecular geometry, also known as the shape of a molecule, is determined by the spatial arrangement of atoms. This is important because the 3D shape affects the physical and chemical properties of a molecule. To predict molecular geometry, we often use the VSEPR (Valence Shell Electron Pair Repulsion) theory, which states that electron pairs around a central atom tend to orient themselves as far apart as possible.
Let's take carbon tetrachloride (\(\mathrm{CCl}_4\)) as an example again. It forms a tetrahedral geometry, where each of the four chlorine atoms is positioned at the corners of a tetrahedron with carbon at the center. This geometry minimizes the repulsion between the pairs of electrons in the carbon-chlorine bonds. Understanding the molecular geometry helps in predicting physical interactions, boiling and melting points, and the overall reactivity of the molecule.

Covalent Bonding

Covalent bonding is the glue that holds atoms together in many molecules. It is the result of atoms sharing pairs of valence electrons to reach a stable electronic configuration. This type of bonding forms between nonmetal atoms, where differences in electronegativity are not too large, leading to shared electron pairs.
For instance, in water (\(\mathrm{H}_2\mathrm{O}\)), each hydrogen atom forms a covalent bond with oxygen by sharing its single valence electron with one of oxygen's six valence electrons. This sharing allows each hydrogen to emulate the nearest noble gas structure while oxygen achieves the octet rule by sharing its remaining valence electrons with two additional electrons from the hydrogen atoms. Recognizing when and how covalent bonding occurs is essential, as it influences molecular stability and interactions in various chemical reactions.